Concrete is widely used in commercial building construction and is present in a number of forms: poured concrete, tilt-up concrete and concrete masonry units (CMU). CMU also comes in a number of different forms: standard block, split-face block and scored block. Because of the dull gray appearance of concrete, however, coatings are usually applied in order to make it more aesthetically pleasing. Coatings also protect concrete by isolating it from the environment and make cleaning an easier task. The selection and use of several prominent coating types used on concrete walls are discussed in this article.
SURFACE PREPARATION
Since most concrete is very porous and does not require roughening of the surface, only a minimal amount of surface preparation is needed to properly prepare the surfaces for painting. The porosity of the substrate generally permits the coating to penetrate into the pores, resulting in a tightly adhered coating system. As a result, surface preparation is conducted primarily to clean the concrete substrate to assure it is acceptable for painting.
Surface Cleaning and Testing. In addition, since concrete is prone to cracking and cannot withstand impacts, repairs are often required. Cracks and spalls are typically repaired prior to painting. While some coating systems do not require crack repair (acrylics, stains, etc.), repair is essential when applying elastomeric paints.
Concrete is cleaned to remove contaminants and then tested for cure and for the presence of moisture. Cleaning removes loose dirt, efflorescence, concrete laitance, dust and other particulate contaminants. Typically, low-pressure water cleaning at 5000 psi or less is used to remove these contaminants. Dwell time must be limited to prevent the introduction of significant quantities of moisture to the concrete surface. If concrete becomes saturated, efflorescence may form. Efflorescence is the migration of a salt to the surface of a porous material, where it forms a crystalline residue or powdery surface layer when water evaporates. In some cases, efflorescence is present prior to the start of a painting project and needs to be removed. This is typically accomplished by cleaning the affected surface with a dilute hydrochloric acid solution assisted by scrubbing and followed by rinsing. Again, water use needs to be limited or the efflorescence will reform.
In some cases, poured concrete walls are sacked after the walls are set. Sacking is used to produce a smooth surface by filling in all pits and air holes. After dampening the surface, and before it dries, a mixture of dry cement and sand is rubbed over it with a wad of burlap or a sponge-rubber float to remove surplus mortar and fill the voids. Usually there is a significant amount of dry sand and cement dust left on the surface after sacking. If this material is not removed prior to painting, catastrophic failures in the form of coating delaminations can occur.
In addition to visible contaminants, non-visible contaminants can interfere with the adhesion of coatings applied over them. These are commonly found on tilt-up and poured-in-place concrete and are present as form-release agents. These agents are sprayed onto the forms used to create the panels or poured-in-place walls to aid in removal of the forms. While many form- release agents dissipate after 28 days, the time required for concrete to cure prior to painting, some do not. Removal is best accomplished by low-pressure water washing. In some cases, however, special additives are required and this complicates the removal process. Painters should inquire prior to painting about the types of form-release agents that were used so that appropriate methods can be selected to remove the substance.
To determine if a form-release agent is present, mist the surface of the concrete with water. If it the water is readily absorbed, the surface is free of bond-breaking contaminants. The water test is very useful even if it is known that a form release agent was not used because, in some cases, densifiers are applied or surfaces are heavily trowelled, preventing free absorption of liquid paint. If concrete will not absorb water, it will also not absorb paint.
Once the cleaning is accomplished, the concrete needs to be tested to verify that it has cured for the necessary period of time. Twenty-eight days has long been the standard in the industry for cure prior to painting. Over this time, the pH of the concrete drops from around 12 to 9 or 10. Typically, when the concrete surface has a pH of 10 or less, it is acceptable to coat. The pH is tested very easily by applying a few drops of water to the surface and then swabbing the water with a pH test strip or applying a pH probe to the surface of the concrete.
Finally, the moisture content needs to be tested. If coating is applied to concrete having too much moisture, blistering of the coating will occur. When heated by the sun, the moisture is converted to vapor, which exerts pressure on the back of the coating, forcing it off of the surface and forming a blister or delamination.
On vertical surfaces moisture testing is typically performed by relative humidity testing or by the plastic-sheet method. Humidity testing is accomplished using instruments that determine the relative humidity either down in the concrete or at the surface of the concrete. Holes are drilled in the concrete and then a tube sleeve is inserted into the hole. A probe is inserted into the sleeve and allowed to equilibrate to the surroundings. Usually, after a 72-hour period, the probe is read using a relative humidity gage. This method is quantitative since it provides the user with an actual number (i.e., 75 percent). Most manufacturers publish upper limits on the humidity their coatings can tolerate.. ASTM F2170 – 11, “Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes” provides guidance for moisture testing using these instruments.
The second test method, the plastic sheet method, is described in ASTM D4263, “Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method. This method is qualitative in that it provides only a go or no-go result. A piece of plastic approximately 18 inches by 18 inches is placed on the surface and the edges sealed to the concrete. The sheet must remain in place overnight and then is removed the next day. If moisture is present on the underside of the plastic, then the concrete is not ready to be painted.
CONCRETE REPAIR
Concrete repair is performed to improve the appearance of the painted concrete substrate and to prevent an excessive amount of water intrusion. Cracks in the concrete will allow wind-driven rain to enter the substrate, resulting in interior damage and possible blistering of the coating system. While some coating systems, such as acrylics, can withstand some moisture intrusion into the concrete, others cannot and will blister if water is allowed to permeate to the back of the coating.
Cracks are typically repaired in two ways. Hairline cracks less than 1/16-inch wide are typically spackled with a knife-grade acrylic caulk. Larger cracks, however, are v-grooved and filled with an acrylic or epoxy polymer-fortified cementitious material.
Spalls are also often repaired with a polymer-fortified cementitious material. In some cases, the spall has to be cut out to create a square edged recession so that the material can be applied at the correct thickness, assuring that the repair material will cure properly and bond to the substrate. Since some of the repair materials also cure by hydration, the repair material needs to be thick enough to hold the amount of water that is necessary to achieve complete hydration. If applied too thin, these materials will be friable and will often crumble after a few weeks of exterior exposure.
CONCRETE COATING SYSTEMS FOR COMMERCIAL PAINTING
As indicated above, coatings are typically applied to concrete surfaces on commercial buildings for aesthetic reasons, but there are a few coating systems that have additional functions, primarily preventing water intrusion. Most coatings are acrylic or modified acrylics and are formulated as standard latex coatings. These are often used in conjunction with block fillers for application to heavily textured surfaces or surfaces having an abundance of bug holes. A subset of the acrylic coatings are the hydrophobic acrylics which repel water and reduce dirt pick-up.
Elastomeric coatings, usually styrene modified acrylics, are used for aesthetic purposes, as well, but also provide barriers against water intrusion. Block fillers are also used with these systems on split faced block prior to the application of the elastomeric coating in order to soften the sharp edges of the surface.
In recent times acrylic stains have been used to color the concrete surfaces. While there is some film build to these stains, the concrete surface is still plainly visible through the coating film. Finally, silane or siloxane sealers are used to seal the concrete and to repel water.
Acrylic Latex Coatings
Acrylic latex coatings are commonly applied to exterior concrete surfaces, primarily for aesthetic reasons. They can be applied by brush, roller or spray at a dry film thickness of around 1 to 3 mils per coat. An acrylic primer is typically applied prior to two coats of the finish paint to achieve a total system thickness of about 3 to 6 mils. The primer is formulated to penetrate into the substrate and seal it. It is also alkali-resistant1 and serves as a barrier protecting the topcoats from degradation as a result of the substrate alkalinity.
The acrylics are also breathable. They allow a certain amount of moisture vapor in the concrete wall to permeate the coating film to prevent blisters from forming. However, excessive amounts of water in the substrate can cause the acrylics to blister. When substrates are heavily cracked, repairs are recommended.Acrylic latex coatings do not serve as effective barrier coats and will allow moisture to permeate through the coating film into the concrete. Moisture can also enter the concrete when acrylics are applied to textured or rough surfaces without the use of a block filler.
Further, acrylics have limited elongation and as a result will not resist cracking. The acrylics have reasonable color and gloss retention, but are inferior to the more expensive two-component polyurethanes or the recently introduced high performance acrylic coatings. The high performance acrylic coatings are reported to have color and gloss retention capabilities consistent with the polyurethane coatings.
Water-Repellant Acrylics
Some of the acrylic coatings are formulated to be water-repellent. That is, wind-driven rain will bead up and run off. In addition, the water-repellent coatings resist dirt pick-up and mold, mildew and algae growth. Standard acrylics can soften when subjected to prolonged moisture exposure. The soft coatings will pick up dirt more readily. The water-repellent acrylics are self-cleaning.
Chalky Surface Acrylic Primers
When overcoating existing coatings, it is important to remove any chalk that is present on the surface. Chalk forms on paint when the binder in the coating (the glue that holds the coating film together) breaks down to release the pigment (chalk). The chalk is a bond breaker and, if present in significant quantities, can result in widespread coating failures in the form of delaminations.
While it is always recommended that all chalk be removed from existing coating systems, sometimes it is difficult to remove completely. In these cases, a chalky-surface acrylic primer should be applied. These primers are formulated to penetrate the chalk and fortify it, improving adhesion of the topcoats.
Block Fillers
Block fillers are used in conjunction with acrylic coatings and are formulated to fill in textures in concrete and CMU. In addition, they will soften the sharp edges of split faced block. The result is a smooth and uniform surface.
Block fillers are applied by brush, roller and airless spray. When spray-applied, block fillers are typically back-rolled to work the coating into the pores of the concrete. If an extremely smooth surface is desired, the applied block filler can be squeegeed. As these coatings are somewhat viscous, they are usually applied at 8 to 10 mils dry film thickness.
Acrylic coatings and block fillers should not be substituted for an elastomeric coating since they do not have the same elongation capabilities as the elastomeric coatings.
Concrete Stains
Until the past few years, staining concrete required acid stains, which were somewhat hazardous to use. Most acid stains are a mixture of water, hydrochloric acid, and acid-soluble metallic salts. They penetrate the surface of the concrete and chemically react with the calcium hydroxide in the concrete. The acidic portion of the stain lightly etches the surface, allowing the metallic salts to penetrate more easily. Once the stain reacts, it becomes a permanent part of the concrete and won’t fade, chip off, or peel away. The acid-based stains were limited to a few colors (tans, browns and soft blue-greens).
However, in the early 2000s, water-based acrylic stains were introduced and are becoming more popular than the mineral acid stains. The water-based stains provide a much wider color range that includes bright colors such as reds and yellows. Like acid stains, the acrylic stains penetrate the concrete to produce permanent color, but no chemical reaction occurs. The acrylic stains can be translucent or opaque. The color is more consistent across the surface than when acid stains are used.
It is important to note that the concrete surfaces still need to be etched using a dilute hydrochloric acid solution prior to the application of the stain. The stains are typically applied by spray and allowed to dry. Additional coats can be applied to deepen the color.
Elastomeric Coatings
Elastomeric coatings are applied for both aesthetics and protection against moisture intrusion. If applied properly, elastomeric coatings form a membrane on the surface of the concrete. In addition, because of the elongation characteristics (up to 700 percent elongation) of the coating, it will bridge hairline cracks if they form after application.
While the coatings are breathable,2 permitting naturally occurring moisture vapor in the wall to escape, excessive amounts of moisture in the concrete will result in blistering. As a result, it is very important that the coating be applied free of holidays, including pinholes. For this reason, the application of sealants to penetrations, edges and changes in materials is essential. Similarly, when elastomeric coatings are applied to split-faced block, block fillers are often recommended prior to elastomeric application.
Along the same line, it is important that cracks and spalls are repaired as described above. Coating will flow away from the sharp edges along crack lines and spalls, resulting in coating holidays that are entry points for water. Horizontal surfaces need to be sloped with a resurfacer in order to prevent water from puddling on the surface. Prolonged exposure to water will soften and degrade the coating.
Note that crack fillers and re-surfacers need to be compatible with the elastomeric coating. Certain fillers and re-surfacers will soften the elastomeric, causing it to pick up dirt preferentially to the surrounding areas.
Elastomeric coatings are applied fairly thick. Typically, two coats are applied at 6 to 10 mils per coat to assure that any pinholes in one coat are covered by another coat. Pinholes form in the coating when air bubbles, entrained in the coating film, do not fully escape. Since the coating is applied fairly thick, the air takes some time to escape. Thus, the coating should not be applied to surfaces in direct sunlight and warm temperatures because the coating may harden before the air escapes.
Elastomeric coatings can be applied by brush, roller or spray. When spray-applied, they are often back-rolled to work the coating into the pores of the concrete, displacing the air and reducing pinholes.
Silane and Siloxane Water Sealants and Repellents
Silane and siloxane water-repellents penetrate into the concrete and chemically react with the concrete down in the pores to form a barrier. This keeps liquid water from entering into the concrete. Also, since the sealer penetrates into the block, it is unaffected by UV rays. Because sealants prevent moisture from entering block, they stop the formation of efflorescence.
The silane and siloxane water-repellents are not visible. Sealed concrete and CMU will have the same appearance as if it were unsealed. There is no gloss, color or visible film. Coatings can be successfully applied over the sealant-treated surfaces.
Sealants are applied by garden sprayer or low pressure airless spray. It is important that they are applied evenly in accordance with the manufacturer’s recommended coverage rates. Prior to application all cracks and joints need to be repaired.
Testing for a successful application should be done by Rilem tube testing. This test involves sealing a Rilem Tube (small water reservoir) to the surface of the concrete or block, filling it with water and monitoring the loss through the block. The silane and siloxane manufacturers have published absorption rates, above which additional applications may be necessary.
SUMMARY
A successful coating project on exterior concrete walls requires attention to a few important considerations. First, make sure that visible and noon-visible contaminants detrimental to coating adhesion are removed. Second, follow through with testing to assure that the concrete can accept the paint, that it is sufficiently dry, and that it is at a pH level suitable for coating. Finally, select a coating material that fulfills the requirements of the job. D+D
1As stated above concrete is alkaline (high pH). Some coatings (alkyds and polyvinyl acetates) will saponify and lose adhesion in the presence of moisture when applied to alkaline surfaces. Some acrylics are also susceptible to degradation (alkaline burn) if applied to concrete.
2Elastomeric coatings will permit naturally occurring moisture vapor in the wall to escape, but not moisture as a result of water intrusion.
ABOUT THE AUTHOR: Ray Tombaugh is a senior consultant and head of the Western Operations of KTA-Tator Inc., a consultancy firm providing coatings failure analysis, condition assessment, specification preparation, expert testimony, project management and other coatings-related services. He is a NACE Certified Coatings Inspector and an SSPC Protective Coatings Specialist. Tombaugh also has a significant amount of experience in historic preservation, serving on the boards of several historic preservation groups in Pennsylvania and New Jersey.